In a conventional step-up converter, an SUC inductor is connected in series with an SUC diode in one of two lines connected to a DC voltage source. Downstream of the SUC diode, an intermediate circuit capacitor is connected between the two lines, to which intermediate circuit capacitor an output voltage, which has been stepped up in comparison with its input voltage by the step-up converter, is applied. Between the SUC inductor and the SUC diode, a shunt branch, in which an SUC switch is arranged, leads from the one line to the other line. Closing the SUC switch applies the input voltage directly to the SUC inductor in order to energize the SUC inductor with a large current flowing through the SUC switch. After the SUC switch has been opened, this energy is output to the intermediate circuit capacitor with a higher voltage by the SUC diode.
A plurality of parallel partial converters are provided in a step-up converter known from DE 101 03 633 A1. The partial converters charge a common intermediate circuit capacitor and each have a series circuit comprising an SUC inductor and an SUC diode and a shunt branch that branches off in between them and has an SUC switch. Therein, the individual SUC switches are each controlled for intermittent operation of the current flowing through the associated SUC inductors. This means that the current through the respective SUC inductor is zero at the time at which the associated SUC switch is closed and returns to zero again after the associated SUC switch has been opened again and before it is closed again. During this intermittent operation, switch-on losses of SUC switches in the form of IGBT semiconductor switches and also switch-off losses of the SUC diodes are minimized in an advantageous way. However, as a result of the fact that high currents are switched off using the SUC switches during intermittent operation, relatively high switch-off losses occur in the SUC switches even if they do not use up the minimization of the other switching losses. Current ripple is reduced in the known step-up converter by controlling the SUC switches of the partial converters connected in parallel in an interleaved manner. That means, despite the intermittent operation of the individual partial converters, the charging current in the intermediate circuit capacitor does not return to zero.
The document EP 1 519 475 A1 discloses a step-up converter having a bypass path that runs parallel to the shunt branch having the SUC switch and, like the latter, branches off between the SUC inductor and the SUC diode. A bypass capacitor for the SUC switch is arranged in the bypass path, the bypass capacitor being connected in series with a diode in the bypass path. When the SUC switch is opened, the current can continue to flow via the diode to the bypass capacitor until the bypass capacitor has been charged to the same voltage as an output-side intermediate circuit capacitor of the step-up converter. Since the voltage applied across the SUC switch is predefined by the voltage applied across the bypass capacitor, the voltage increase across the SUC switch is limited. In order to discharge the bypass capacitor for the next switching-off operation and to supply the electrical energy stored in the bypass capacitor to the intermediate circuit capacitor, a discharge circuit having a controllable switch is provided. The switch of the discharge circuit is controlled in such a manner that it is closed when the SUC switch is closed before switching off the current.
US 2006/0274558 A1 discloses a step-up converter having a so-called snubber circuit. The step-up converter has two input terminals and two output terminals, an SUC inductor and an SUC diode being connected in series between the first input terminal and the first output terminal, and a shunt branch having an SUC switch branching off between the SUC inductor and the SUC diode. The snubber circuit has a path comprising a first diode, a second diode and an inductor that are connected in series between the first output terminal and the second input terminal of the step-up converter. The inductor of the snubber circuit is magnetically coupled to the SUC inductor, and the forward directions of the two diodes of the snubber circuit are the same. The snubber circuit also has a capacitor that is connected in series with the first diode of the snubber circuit between the first output terminal of the step-up converter and a node between the SUC inductor and the SUC diode. When the SUC switch is closed, the magnetic coupling of the inductor to the SUC inductor causes a current flow in the first path of the snubber circuit that charges the capacitor, the polarity of the resulting voltage across the capacitor being opposite to the polarity of the voltage across an intermediate circuit capacitor between the output terminals. When the SUC switch is opened again, the interrupted current flows to the capacitor, as a result of which the latter is discharged. Since the voltage applied across the SUC switch cannot exceed the difference between the voltage applied between the output terminals and the voltage applied across the capacitor, the occurrence of overvoltages at the SUC switch is counteracted.
In a conventional step-down converter, an SDC switch is connected in series with an SDC inductor in one of two lines connected to a DC voltage source. Downstream of the SDC inductor, an intermediate circuit capacitor is connected between the two lines, to which intermediate circuit capacitor an output voltage, which has been reduced in comparison with its input voltage by the step-down converter, is applied. Between the SDC switch and the SDC inductor, a shunt branch, in which an SDC diode is arranged, leads from the one line to the other line. Closing the SDC switch causes the flow of a limited current, which energizes the SDC inductor, through the SDC inductor to one side of the intermediate circuit capacitor. After the SDC switch has been opened, this energy is output via the SDC diode in order to charge the intermediate circuit capacitor further.
A step-up converter is known from US 2006/0262577 A1. Here, a saturable inductor is connected between the SUC inductor and the SUC diode and downstream of the shunt branch having the SUC switch. A discharge circuit for a bypass capacitor connected in series with a diode and in parallel to the SUC switch has an inductor that is connected to the same connection of the bypass capacitor as the diode connected in series therewith, and a further diode connected in series with the inductor. The two diodes have forward directions that are opposite one another, as seen from the bypass capacitor. The further diode leads, on the one hand, to a resonant capacitor, which forms a series resonant circuit together with the bypass capacitor and the inductor and the other end of which is connected between the saturable inductor and the SUC diode, and, on the other hand, to a discharge diode that is connected to the output of the step-up converter downstream of the SUC diode. The bypass capacitor is charged when the SUC switch is opened and is discharged into the resonant capacitor within half a resonance period of the series resonant circuit while the SUC switch is closed. The resonant capacitor, from which the charge cannot flow back to the bypass capacitor due to the interposed further diode, is discharged when the SUC switch is opened again, in which case the bypass capacitor is then also charged again.